1. Field of the Invention
This invention relates to semiconductor memory devices and more particularly to a MOS type READ ONLY MEMORY (ROM) device whose programmed cells of the memory array are determined by the gate configuration of the cell.
2. Description of the Prior Art
In READ ONLY MEMORY (ROM) devices, by definition, the device is fabricated with a predetermined custom ROM pattern of logical ones and logical zeros. Prior art ROM devices are disclosed in U.S. Pat. No. 3,191,151 entitled "Programmable Circuit", which issued June 22, 1965 to Price. Typically, diffused buried bit lines and overlying perpendicular conductive word lines are constructed on a semiconductor substrate wherein the word line provides a gate for each cell formed at the crossover intersection of a word line and a bit line (FIG. 2). To provide the ROM function of the device, certain of the memory cells must be permanently "off" or in the logical "one" condition while the other cells are capable of being turned on and off, or in the logical "zero" condition. Heretofore, in semiconductor ROM devices made according to conventional metal-oxide semiconductor (MOS) processes using the well-known metal gate technique, programmed cells are formed during a step fairly late in the process. An advantage in this process is that the manufacturer of such ROM devices could build an inventory of nearly finished parts and then perform the final steps when necessary to complete devices with a custom ROM pattern.
Because of other inherent advantages, such as greater speed, lower power consumption and smaller physical dimensions, the well-known silicon gate devices were later developed and are preferred over metal gate devices for certain applications. However, the silicon gate process requires the ROM pattern of active and inactive cells to be formed at a relatively early stage in the series of process steps, which eliminates the production advantage of metal gate ROM devices.
There thus arose a need for an improved MOS structure and process for making ROM devices that would retain the inherent advantages of silicon gate devices and yet allow the ROM to be comitted to a specific program pattern only after at least 50 to 60 percent of the process had been completed.
One general object of the present invention is to provide an improved metal-oxide semiconductor ROM device that solves the aforesaid problem.
Another object of the invention is to provide a semi-conductor ROM device wherein the active and inactive cells once formed have a different geometrical configuration so that the ROM pattern for the device can be visually ascertained and checked. This cannot be done in many types of prior art ROMs.
Still another object of the invention is to provide a method for making an improved metal-oxide semiconductor device that has lower power characteristics and is particularly adaptable for implementation as a memory device.